1. Field of the Invention
The present invention relates generally to the field of wireless communication and more particularly to a method and apparatus for enhancing the throughput of a wireless communication system by using multipath transmission.
2. Description of the Related Art
Generally when radio waves leave a wireless radio transmitter, they spread out in all directions away from the transmitter. There is usually a direct path from the transmitting station to a receiving station. Some of the transmitted radio energy follows this direct path; however, a portion of the energy follows other paths. The phenomenon of more than one path for radio transmission between two stations is called multipath. Each path is called a multipath component. In particular, multipath is the arrival at the receiver of radio waves from the transmitter that followed various paths from the transmitter to the receiver. The reason for this effect is that radio waves reflect from numerous objects including buildings, cars, airplanes, trees and almost any other object. Thus many possible reflective paths may exist that allow a wave to leave the transmitter at some departure angle away from the direct path, reflect off of buildings, etc. and finally arrive at the receiver at a slightly delayed time from the waves following the direct path. There may be multipath components that have made several reflections before reaching the receiving station. Each different multipath component has a unique angle of departure from the transmitting antenna, a unique angle of arrival at the receiving antenna, and a unique path delay. Path delay at the speed of light is around 1 nanosecond per foot of path. Thus a radio wave arriving on a path that was 1000 feet longer than the direct path will arrive 1 microsecond later than the direct wave.
In the past, multipath has been considered a nuisance in wireless communications. The reason for this is that multipath reception many times causes deep signal fading and unwanted interference. Fading is caused when the signals along the direct path and the secondary path cancel (because the secondary path is delayed 180 degrees in phase at the transmission frequency). Interference is caused by smearing of several replicas of the transmitted signal arriving at the receiver.
It has been suggested that these problems can be overcome if the multipaths can be mapped out, and the direct path signal delayed slightly so that the multipath components arrive in phase. In fact, it is known in the art to map wireless channels between two points and provide correct delays on several multipath components to improve communication of a single signal. It is also known to use several different transmit and receive antennas between two points where the antennas use different paths. This is called space diversity, and it is known to improve communications quality. It is also known to provide different types of modulation and data coding that relies on space diversity (or so called space-time diversity) (See IEEE Spectrum, June 2002, p. 40).
It is not necessary to transmit the same signal on each of the multipath routes between a transmitting station and a receiving station. Different information can be transmitted on different pairs of transmit/receive antennas to increase the data throughput rate rather than provide space diversity. U.S. Pat. No. 6,285,720 teaches a method and apparatus for high data rate wireless communication over wavefield spaces. Here, multiple space or path channels that share the same time and frequency between the transmitter and the receiver are used to transmit different information. A different antenna and receiver front-end at the receiving location was used for each of these different paths.
The problem with this approach to increasing throughput between a transmitting and receiving station is that it requires several highly directional antennas at both the transmitting location and at the receiving location. Alternatively phased arrays or “smart antennas” can be used, but these are large, complex and expensive. One example of a wireless link is between a base station and a hand-held mobile unit (such as a cellular telephone link). In such communication systems, the hand-held unit must be small and cheap. It is almost impossible for the hand-held unit to have more than one antenna or any kind of “smart antenna” because antenna arrays are quite large. A hand-held unit's antenna is usually a vertically polarized omni-directional rod or dipole. This type of antenna combines all multipath components arriving into a single signal.
Also known in the art is the “BLAST” system developed by Lucent Technologies Bell Laboratories, and other similar systems, that transmit different signals on N different antennas. These signals are received on M different receiving antennas. This system relies on the different signals spatial characteristics to provide separation. It does not solve the problem of increasing the data rate of a channel to a small hand-held receiver. The performance of the “BLAST” system is known to depend on N and M being generally greater than 1 with optimums being around 5-15 and with very sophisticated signal processing at the M receivers and the combiner. The “BLAST” system, as proposed, uses a single spreading code on all channels (See Huang and Viswanathan, “Multiple Antennas in Cellular CDMA Systems: Transmission, Detection, and Spectral Efficiency”, IEEE Transactions on Wireless Communications, Vol. 1, No. 3, July 2002).
It is desirable to have a method and apparatus to transmit different information on different multipath components between a transmitting station and a receiving station where the receiving station can be a small hand-held unit with a single omni-directional antenna (such as a cellular telephone would have). By carefully choosing which multipath components to use between two stations, such a system could multiply the amount of data transmitted in a given period of time thereby increasing the throughput to a very small receiver while simultaneously avoiding the problem of multipath fading.